An example of a conventional volume control of this type is shown in FIG. 5. It includes operational amplifier OA with noninverting signal input + connected with input signal connection E of the volume control, and inverting feedback input - and signal output A. Between input signal connection E and noninverting signal input + of operational amplifier OA there is switching point P connected via resistor R with signal ground connection SGND. Between output connection A and inverting feedback input - there is a feedback circuit with two cascade-connected resistor strings or voltage dividers ATT2 and ATT16. These two voltage dividers ATT2 and ATT16 have not only a different number of component voltage taps but different component voltage gradations. While ATT2 has eight component voltage taps and a component voltage gradation of 2 dB between adjacent component voltage taps in the shown embodiment, ATT16 has four component voltage taps and a component voltage gradation of 16 dB between adjacent component voltage taps. ATT2 thus forms a voltage divider with fine gradation while ATT16 is a voltage divider with coarse gradation.
ATT2 is connected on its high-voltage side with output A of operational amplifier OA and on its low-voltage side with signal ground connection SGND. Each of the component voltage taps of ATT2 is connected via controllable electronic switch S21 to S28 with the high-voltage side of ATT16, whose low-voltage side is likewise connected with signal ground connection SGND. FIG. 5 shows only switch S21 associated with the uppermost component voltage tap of ATT2. The other seven switches S22 to S28 are not drawn but only indicated by their respective switch reference signs. All these eight switches S21 to S28 are connected on their side remote from ATT2 jointly with the high-voltage side of ATT16.
Associated in turn with the four component voltage taps of ATT16 are controllable electronic switches S31, S32, S33, S34, respectively, whereby again only uppermost switch S31 is shown and the other three switches are only indicated by their reference signs S32 to S34. The sides of switches S31 to S34 remote from ATT16 are connected jointly with feedback input - of OA.
The switching states of switches S21 to S28 are controlled by a digital switch control circuit, first decoder D1, while the switching states of switches S31 to S34 are controlled by another digital switch control circuit, second decoder D2. In the shown embodiment D1 is a 3/8 decoder which converts 3-bit control data words supplied to its input side into switch control signals which it can conduct via eight output lines to gates of electronic switches S21 to S28. In accordance with the data content of the particular 3-bit control data word, this renders a selected one of the eight switches S21 to S28 conductive while rendering the others nonconductive. In accordance with the control data value of the particular control data word, one of the component voltage taps of ATT2 is therefore connected with the high-voltage side of ATT16.
In corresponding fashion, the control through second decoder D2 formed as a 2/4 decoder renders one of the four switches S31 to S34 associated with second voltage divider ATT16 conductive while rendering the other three switches nonconductive, so that feedback input - of operational amplifier OA is connected with one of the four component voltage taps of ATT16 in accordance with the 2-bit control data word supplied to D2.
In this way the digital switch control signals supplied to decoders D1 and D2 serve to adjust a very definite feedback for operational amplifier OA, which leads to a certain gain of the audio signal supplied to input signal connection E.
The symbols used in FIG. 5 for switches S21 and S31 stand for electronic switches having a configuration shown in FIG. 2. These switches consist of a parallel connection of an NMOS transistor having an N-channel and a PMOS transistor having a P-channel. The gate electrodes of NMOS and PMOS are connected with control signal input SE, the gate electrode of NMOS directly and the gate electrode of PMOS via inverter INV. Therefore both transistors NMOS and PMOS of the electronic switch are always rendered conductive or nonconductive depending on the type of control signal supplied to control signal input SE. Control signal input SE is connected with a corresponding output signal line of first decoder D1 or second decoder D2. In FIG. 5 the eight and four output control lines of D1 and D2, respectively, are only shown in a simplified symbolical way by one dashed line in each case.
The use of electronic switches with the configuration shown in FIG. 2 is known in the art. This type of switch is used in order to obtain better linearity than can be obtained using only one switching transistor. But this switch constructed from two parallel-connected transistors also has higher nonlinearity than is desirable for high-quality audio devices.
FIG. 3 shows the characteristic of on resistance R.sub.ON of a switch of the type shown in FIG. 2 as a function of input voltage V across this switch. One can see that this switch also has considerable nonlinearity despite the use of the two parallel-connected transistors NMOS and PMOS. This resistance characteristic has the lowest nonlinearity in its middle area. Therefore one places the switch-on operating point of this electronic switch so that it is in the middle between the two "humps" in the resistance characteristic. One does so by setting the operating point at VS/2, whereby VS is the supply voltage of the integrated circuit. As the sinusoidal signal indicated in FIG. 3 shows, it is subject in the switch shown in FIG. 2 to a nonlinear transmission, which leads to distortion with a factor depending on the nonlinearity of the characteristic.
The invention is intended to obtain a compensation of the distortion caused by this nonlinearity.